When shopping for vacuums, you may encounter the following specifications.
Airflow and static lift. Airflow is expressed in cubic feet per minute (cfm) and is a measure of how much air the vacuum moves. It's a particularly important spec because it indicates the vacuum's ability to move light materials like dust – which is what the RRP focuses on.
Static lift is a measure of suction force and is expressed as the height of a column of water (in inches) that the vacuum can lift; it's a good indication of how the vacuum will perform when lifting heavy objects (like liquids). While there is some value in being able to pick up heavy materials, HEPA is not required when collecting liquids and most contractors will not want to subject an expensive HEPA machine to liquid-extraction tasks.
Motor rating. Horsepower and wattage tell you something about the motor but do not describe how the vacuum performs – static lift and airflow are far better measures of that. Wattage may be an issue with tool-activated vacuums, because if the combined wattage of the vacuum and the tool is too high it could trip the circuit breaker. To prevent this from happening, a tool-activated vacuum is typically equipped with a soft-start switch or a current-monitoring device that slows the motor when the combined draw approaches 15 or 20 amps.
Weight. Weight is important if you buy a backpack model. It's less of a factor with floor models because they usually have wheels, though you ought to consider what it is like to lift the machine out of a vehicle and carry it into the building.
Capacity. Capacity is measured in gallons and is the volume of material the vacuum can hold. High-capacity vacuums take up more space in your vehicle and can be heavy when full. Low-capacity models are easier to transport but must be emptied more frequently.